``The X 1A1, a 3B1, A 1B1, and B 1A1 Electronic States of SiH2,'' Y. Yamaguchi, T. J. Van Huis, C. D. Sherrill and H. F. Schaefer, Theor. Chem. Acc. 97, 341-349 (1997).

Four electronically low-lying states of silylene (SiH2) have been studied systematically using high level ab initio electronic structure theory. Self-consistent-field (SCF), two-configuration (TC) SCF, complete active space (CAS) SCF, configuration interaction with single and double excitations (CISD), and CASSCF second-order (SO) CI levels of theory were employed with eight distinct basis sets. The zeroth-order wave functions of the ground (X 1A1 or 1 1A1) and B 1A1 (or 2 1A1) excited states are appropriately described by the first and second eigenvectors of the TCSCF secular equations. The TCSCF-CISD, CASSCF, and CASSCF-SOCI wave functions for the B 1A1 (or 2 1A1) state were obtained by following the second root of the CISD, CASSCF, and SOCI Hamiltonian matrices. At the highest level of theory, the CASSCF-SOCI method with the triple zeta plus triple polarization augmented with two sets of higher angular momentum functions and two sets of diffuse functions basis set [TZ3P(2f,2d)+2diff], the energy separation (T0) between the ground (X 1A1) and first excited (a 3B1) states is determined to be 20.5 kcal/mol (0.8990 eV, 7180 cm-1), which is in excellent agreement with the experimental T0 value of 21.0 kcal/mol (0.910 eV, 7340 cm-1). With the same method the T0 value for the A 1B1 -- X 1A1 separation is predicted to be 45.1 kcal/mol (1.957 eV, 15 780 cm-1), which is also in fine agreement with the experimental value of 44.4 kcal/mol (1.925 eV, 15 530 cm-1). The T0 value for the B 1A1 -- X 1A1 separation is determined to be 79.6 kcal/mol (3.452 eV, 27 840 cm-1). After comparison of theoretical and experimental T0 values for the a 3B1 and A 1B1 states and previous studies, error bars for the B 1A1 state are estimated to be +/- 1.5 kcal/mol (+/- 525 cm-1). The predicted geometry of the B 1A1 state is re(SiH) = 1.458 Angstrom and Thetae = 162.3o. The physical properties including harmonic vibrational frequencies of the B 1A1 state are newly determined.